The present disclosure relates to nitride semiconductor devices and methods for fabricating the nitride semiconductor devices. More particularly, the present disclosure relates to a nitride semiconductor device which requires a selective etching process and a method for fabricating the nitride semiconductor device.
Nitride semiconductors, such as gallium nitride (GaN), have a large band gap and therefore a large breakdown field strength and a high saturated drift velocity. Therefore, in recent years, nitride semiconductors have been widely studied and developed as materials for blue-violet semiconductor laser devices serving as light sources for recording and reproduction of high-density optical disks capable of high-density information recording and reproduction, materials for high frequency and high power semiconductor devices, etc. Nitride semiconductors belong to the same group III-V semiconductors including aluminum gallium arsenide (AlGaAs) used as a material for red laser devices, high-frequency semiconductor devices, etc., and indium gallium phosphide (InGaP) used as a material for infrared laser devices. However, nitride semiconductors have characteristics significantly different from those of AlGaAs and InGaP, and therefore, existing techniques are not directly applicable. Therefore, the fabrication of semiconductor devices made of nitride semiconductors suffers from low yield. Technological advances are desired which increase the yield of semiconductor devices made of nitride semiconductors.
The fabrication of semiconductor devices made of nitride semiconductors requires a selective etching process. For example, it is known that when a field effect transistor (FET) is formed by stacking a GaN layer and an AlGaN layer, a gate recess structure is formed by selectively etching a portion of the AlGaN layer in order to reduce a gate leakage current. In this case, it is necessary to selectively remove the AlGaN layer with high accuracy. Also, when a buried semiconductor laser device is formed of a nitride semiconductor, it is necessary to selectively remove a current confinement layer with high accuracy.
Dry etching is relatively easily performed on nitride semiconductors. However, dry etching has poor controllability of film thickness. Therefore, for example, when the gate recess structure is formed, it is difficult to accurately stop the etching process so that the AlGaN layer having a predetermined thickness is left. If dry etching reaches an interface between the AlGaN layer and the GaN layer, a channel which is a passage for electrons is not formed, so that the FET does not work. Also, if the amount of the remaining AlGaN layer varies, characteristics of the FET also varies. Moreover, dry etching has a drawback that a large amount of crystal defects are generated. These problems arise not only when the gate recess structure is formed, but also when a current path is formed in a buried semiconductor laser device.
On the other hand, wet etching advantageously has good controllability of film thickness, and is not substantially accompanied by crystal defects. Therefore, techniques of fabricating a blue-violet laser device by wet etching an amorphous or polycrystalline nitride semiconductor have been described (see, for example, Japanese Patent Publication No. H11-261160).
However, there is a problem that it is not easy to wet etch crystalline nitride semiconductor layers. Therefore, when it is necessary to process a crystalline nitride semiconductor layer, an amorphous or polycrystalline nitride semiconductor layer may be wet etched before crystallization by a thermal treatment. It is, however, difficult to obtain a nitride semiconductor layer having good crystallinity using such a technique. A technique of accurately processing a crystalline nitride semiconductor by direct wet etching is desired.
Even if wet etching is employed, perfect etch selectivity is not obtained, i.e., etching residue or overetching occurs. There is a demand for a technique of stopping etching accurately at an interface in order to improve the performance of a semiconductor device.